Thursday’s New Moon Raises Higher Tides, Double Shadows on Jupiter, and Measuring the Sky using the Big Dipper!

Star Walk
8 min readMay 22, 2017


Public Events

At 7:30 pm on Wednesday, May 24, the RASC Toronto Centre will hold their free monthly Recreational Astronomy Night meeting at the Ontario Science Centre. The public are welcome. Talks include the Sky This Month, Project Starshot to Alpha Centauri, collecting meteorites, and making solar filters for this summer’s eclipse. Check here for details. Parking is free.

The astronomers of York Region Astronomy will hold a free stargazing session at the Bayview Reservoir Park Playground off Sycamore Drive in Thornhill on Saturday evening, May 27th, starting at 9 pm. Visit for the GO/NO-GO call and more information.

The Moon and Planets

As this week opens, the moon is passing through the pre-dawn planets as it swings eastwards towards the sun. On Monday morning between 4 am and dawn, the old crescent moon will be a lovely sight only a few degrees to the lower right (southwest) of Venus. Because the sun is illuminating the moon and Venus from the lower left, they both show crescent phases (but your telescope may flip Venus around). You should be able to use the visible moon during Monday morning to find Venus in daytime. It will show up as a bright pinprick of light a few finger widths higher than the moon. (The rotating Earth and sky will shift Venus to the moon’s upper right by late morning.)

On Tuesday morning, the moon lands 9° (about a clenched fist width), to the right of Mercury. (The moon is slightly higher, too.) While the moon will be easily spotted after it rises at 4:30 am local time, the best time to see Mercury will be around 5:15 am, soon after it has cleared the eastern horizon. By 5:30 am, the brightening sky will be overwhelming the elusive planet. You can see Mercury in the same location all week — low in the west about 2.5 outstretched fist diameters to the south of where the sun will rise. You need a very low horizon free of trees, etc. because, at 5:15 am, the planet is only a few finger widths above the true horizon.

On Thursday afternoon, New Moon phase formally occurs — the moment when the moon’s orbit carries it between the earth and sun, and it’s in the same region of the sky where the sun is. Sunlight is only reaching the side of the moon that is turned away from us, so it can’t be seen. The moon’s orbital tilt is 5° from ecliptic, the plane defining Earth’s orbit around the sun, so the moon usually passes above or below the sun. (Solar eclipses occur when the moon is close to the ecliptic at new moon — as it will be on August 21 this year.) This new moon also happens while the moon is close to perigee — its closest point to Earth. Everywhere on Earth, the combination of the nearby moon, plus both sun and moon in the same direction of sky, will raise higher than average tides on the dates surrounding Thursday.

New Moon in Solar Walk 2 app.

On Friday after sunset, try to catch a glimpse of the slender sliver waxing crescent moon, low near the western horizon. Dim reddish Mars will be a palm’s width to the upper right of the moon. This week, it’s setting about 10:20 pm local time, but the red planet is embedded in the western twilight glow. Finally, on Saturday and Sunday, the waxing crescent moon will travel eastward through the constellation of Gemini (the Twins), landing amid the stars marking the twins’ feet on Saturday.

Yellowish Saturn rises just before 10:30 pm local time this week, and remains visible until hidden by the brightening dawn around 5 am, when it’s two fist diameters above the southwestern horizon. The only other bright object near the ringed planet is the red-orange star Antares sitting almost two fist diameters to the right (west).

Jupiter is the brightest object in the evening sky this week. As darkness sets in, it is halfway up the southern sky, and it sets in the west during the wee hours of the night, local time. Jupiter’s four large Galilean moons are easily visible in a small telescope. A larger telescope will also show the Great Red Spot and the round black shadows cast by Jupiter’s moons when they cross (or transit) the planet. Here are the best events in Eastern Daylight Savings Time. (Simply add or subtract the appropriate hours to convert them to your time zone.)

Jupiter in Star Walk 2 app.

Another great Jupiter double shadow event occurs in the wee hours on Friday, May 26. Europa’s shadow starts across at 12:54 am. Io’s shadow joins it starting at 1:47 am. The two shadows cross for about 90 minutes, until Europa’s moves off about 3:15 am (as Jupiter begins to set for the Eastern Time Zone). Observers farther west see the event higher in their sky.

Io’s shadow crosses alone on Saturday, May 27, already halfway across at dusk and lasting until 10:25 pm.

The Great Red Spot is visible on Jupiter for about three hours centred on Tuesday, May 23 at 1:03 am, and Tuesday evening at 8:54 pm (in twilight), Thursday, May 25 at 2:41 am, Thursday evening at 10:32 pm, and Sunday, May 28 at 12:11 am.

The Great Red Spot in Solar Walk 2 app.

Dim, reddish Mars is now sinking into the evening twilight, setting about 10:30 pm local time this week. It will soon be lost from view while it passes the sun. It late summer it will re-appear in the morning sky — on the way to a terrific viewing opportunity next summer. I’ll post diagrams for the morning and evening planets here.

Measuring the Sky with the Big Dipper

This time of year, the Big Dipper is nearly directly overhead in the evening. The seven bright stars, in order from the tip of the handle, are named Alkaid, Mizar, Alioth, Megrez, Phad, Merak, and Dubhe (“dooby”). Mizar has a tiny partner named Alcor close beside it, usually visible with sharp unaided eyes.

Merak and Dubhe mark the bottom and outer lip of the dipper’s bowl, respectively. If you follow the line connecting them, up and out of the bowl, the next readily visible star that the line passes through is Polaris, the North Star. This little trick to find north works in the northern hemisphere all around the world. Polaris also marks the tip of the curved handle of the Little Dipper, but the other six stars in it are too dim to easily see from the city. The Little Dipper’s bowl is opposite the Big Dipper’s, and its handle droops downwards, while the Big Dipper’s handle is currently upwards.

Astronomers describe how far apart two objects in the sky are by measuring the number of degrees apart they are, or their angular separation. Since the sky is a great bowl, all the measurements are portions (or arcs) of a circle. There are 360 degrees ( ° ) in a circle and therefore 180 degrees in half the sky (horizon to horizon). If you point at two different objects at the same time, the angle between your straight arms is their angular separation.

For close together objects, we divide the degree into sixty arc-minutes ( ‘ ), and each arc-minute into sixty arc-seconds ( “ ). The Full Moon is 30 arc-minutes across, and planets’ disk sizes are given in arc-seconds. Right now, Jupiter is about 42 arc-seconds across.

You can use the Big Dipper to practice how to specify how far apart objects are. The depth of the bowl is 5°. For most people, that’s equivalent to three or four finger widths (with your hand at arm’s length). The width across the bowl is 10°, or about a clenched fist’s width. The length of the dipper from Alkaid at the handle-tip to Dubhe at the far rim, is 25°. Most people can span this with a surfer’s salute — outstretched thumb-tip to pinky fingertip. I’ll post a diagram on my Tumblr version of these Skylights here.

Your little finger is about one degree wide at arms’ length. Remember that the Full Moon is only half a degree across (30 arc-minutes), so even your pinky fingernail can fully cover it. Try it when the moon returns to view next week!

Binocular Comet Update

The new moon darkness this week aids in looking for comets. I posted finder charts for the paths of several visible ones during May here. In binoculars and low power telescopes, expect the comets to appear as faint greenish blobs (quite different from a star). If a comet develops a tail, it will point roughly away from the Sun. If you find one in a telescope, watch for 15 minutes or so — you’ll see it moving with respect to the stars nearby.

Comet 41P/Tuttle-Giacobini-Kresak is visible all night, but it’s highest in the sky before dawn. This week, look in the eastern evening sky to the lower right of the very bright star Vega. It’s dropping lower and westward, increasing the distance from Vega from 16° to 22° over the course of the week.

Comet 41P/Tuttle-Giacobini-Kresak in Star Walk 2 app.

Comet C/2015 V2 (Johnson) is an all-night binocular comet that is still brightening — visible in the southeastern sky as soon as it’s fully dark. It is still moving south through the constellation of Bootes (the Herdsman) in a direction towards that constellation’s brightest star, Arcturus. Look for it about a palm’s width above the circle of stars forming Corona Borealis (Northern Crown).

Although it is bright enough, Comet C/2015 ER61 (PANSTARRS) is getting too low in the pre-dawn eastern sky to see easily. I’ll post some gorgeous photos of the comets here.

Stargazing News for this week (from May 21st) by Chris Vaughan.



Star Walk

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